It is essential with the injection molding and continuous vulcanization of rubber articles that the moldable rubber in the extruder and in the conveyor to the mould remain processable, i.e. readily flowable, as long as possible. In the mould, however, the rubber should crosslink as rapidly as possible. Faster crosslinking permits a higher rate of production.
Since the viscosity of the moldable rubber decreases with higher temperatures, it is desirable to maintain the temperature in the extruder and the conveyor to the mould as high as possible without risking premature crosslinking. In practice, the maximum processing temperature in the extruder/conveyor is at least 30.degree. to 50.degree. C. below the temperature in the mould (vulcanization temperature). A small increase in the processing temperature and/or extension of the residence time in the extruder/conveyor can, above a certain critical level (or threshold-value), lead to premature crosslinking, which is characterized by wrinkled or "scorched" spots on the smooth surface of the rubber article produced. This phenomenon is commonly known as "scorch".
The tendency of a rubber to scorch under commercial operating conditions may be measured by means of an oscillating disc curemeter, which procedure is described in International Standard ISO 3417. A test piece of rubber is maintained at an elevated temperature. A biconical disc is embedded in the test piece and is oscillated. This action exerts a shear strain on the test piece and the force (torque) required to oscillate the disc depends on the degree of vulcanization of the rubber at the elevated temperature at a certain time. The torque is recorded as a function of time.
Indicative of the scorch time of a rubber is the t.sub.2 2 value, which is the time for the torque to increase 0.1 Nm above the minimum torque, measured in the curemeter. Indicative of the necessary residence time in the mould is the t.sub.90 value (vulcanization time), which is the time required to obtain 90% of full torque development. The higher the t.sub.90 value, the longer the residence time in the mould. The final properties of the rubber article depend, to a significant extent, on the crosslink density (degree of crosslinking). Indicative of the crosslink density is the highest torque measured in the curemeter. Another way of measuring the above-mentioned values is described in International Standard ISO 6502.
Until now, the scorching problem has been solved at the expense of either a significantly longer production time (t.sub.90) or an undesirable worsening of the properties of the rubber compositions. For example, typical antioxidants can be used to lengthen the scorch time but invariably lead to a significantly lower crosslink density in the rubber product. It is also possible to extend the scorch time by, for example, adding initiators which have a longer half-life period. Such alternative, however, also results in a longer crosslinking period and hence an undesired lowering of the production rate. Combinations of some coagents with antioxidants have been tried, but all have one or more disadvantages in either compression set, crosslink density or production time.
EP 0 346 863 describes polymer compositions containing an organic peroxide, a hydroquinone derivative, and a coagent such as triallyl cyanurate to extend the scorch time (t.sub.2 2).
U.S. Pat. No. 4 857 571 describes a crosslinkable rubber to which a peroxide, a coagent, such as acrylate, methacrylate, or triallyl cyanurate, and a special scorch retardant containing a phenol group and an amino group, have been added.
FR 1 259 094 describes the use of bismaleimides and biscitraconimides in the vulcanization of saturated polymers in the presence of organic peroxides. Although it is possible to add other vulcanization agents no details are given. Furthermore, this publication does not teach or suggest the process of the present invention nor the advantages which flow therefrom.
In CA 738 500 rubber vulcanization processes are disclosed wherein bismaleimides and biscitraconimides are employed as vulcanization agents in the absence of free radical initiators, optionally in the presence of thiazole type accelerators. The rubbers obtained by this process are said to have a better resistance to oxidative aging than sulfur-vulcanized rubbers. In addition, rubber stocks containing the bismaleimides and biscitraconimides are usable in the presence of metals which are normally tarnished by those stocks and can be processed at higher temperatures than sulfur-containing rubber stocks without scorching. CA 738 500 also discloses a process for the vulcanization of polyurethane rubbers using a bismaleimide or biscitraconimide curing agent and a free radical initiator. Polyurethane rubber vulcanizates cured in this manner exhibit a higher tear strength, better resistance to aging and a higher tensile strength at high temperature. Although this Canadian patent mentions that antioxidants may be used in the vulcanization processes, no specific details are given. Further, this patent does not teach or suggest the process of the present invention or the advantages which flow therefrom.
U.S. Pat. No. 4 018 852 relates to crosslinkable polyethylene compositions comprising an organic peroxide, an organic hydroperoxide and a triallyl compound This patent provides a detailed discussion of the desirability of reducing scorching by extending the scorch time, as well as the desirability to maintain a relatively short production time and good physical properties in the rubber product.
The present approach to achieve long scorch times in peroxide vulcanization is to have the highest possible t2:t90 ratio (scorch ratio), without any of the other physical properties of the rubber being significantly altered.
Accordingly, it remains desirable to lengthen the scorch time in peroxide rubber vulcanization processes without significantly lengthening production time or negatively influencing the advantageous properties of the rubber product.
The present invention provides a solution to the above problems by the use of a combination of a biscitraconimido compound and a radical scavenger in the peroxide-vulcanization of rubbers.